This invention is directed generally to a system for overlaying a first video signal with a second video signal for presentation on a display device and particularly relates to a system for synchronizing first and second horizontal synchronization signals in a video display for overlaying two video signals for simultaneous presentation thereon.
The signal driving a video display is comprised of a video component and a synchronization component. The video signal portion includes picture information such as luminance and chrominance signals. The synchronization signal portion includes pulse-like signals occurring both at the horizontal and vertical scan rates which are interspersed between the scan interval of the picture component in the period generally referred to as the retrace interval. In addition, the sync pulse signals coincide in time with blanking signals used to blank out the video display during electron beam retrace.
In the case of a television receiver, the composite video and synchronization signals are received from an external signal source such as a broadcast signal transmitter. In the case of a video display as utilized in the terminal of a video game, minicomputer, word processor, or any of the ever-increasing applications in which video displays are employed, the video and synchronization signals are typically generated within the unit itself and provided to the cathode ray tube (CRT) of the terminal in response to user inputs. The overlaying of one video image upon another video image in a single display device is becoming increasingly more popular in performing a variety of functions. This type of image overlaying may be used to provide text information superimposed upon the video display of a television receiver, to overlay various images such as that of a game board upon the image displayed on a television receiver to permit the viewer to play a video game while simultaneously viewing the transmitted signal, or overlaying various images from several signal sources on a single video display for artistic, technical or design analysis and evaluation.
When the video image of a first video display such as a conventional television receiver is overlayed by a second video display such as generated by a video display processor, frequently incompatibilities between the two systems result in a degraded composite video image. This typically arises because the horizontal synchronization (sync) signal in the television receiver is modulated by the television receiver's vertical deflection signal which is generally unstable and subject to slight jitter or modulation. The extent of jitter in the horizontal sync signal of the television receiver may frequently exceed the dimensions of an elemental area (pixel) of an image on the second video display thus making precise and stable image overlay impossible. In addition, the video display clock utilized for generating these elemental areas of an image presented thereon is asynchronous and thus may generate a non-integer number of dots or pixels per active scan line. This undesirable effect is visible to the viewer as an image having irregular, or "zippered" edges. Laterally displaced pixels between adjacent horizontal scan lines also result in indistinct images which also may appear to be subject to a "tearing" effect.
The present invention is intended to avoid these limitations by providing a stabilized composite sync signal and a phase locked pixel or dot clock derived from a first video signal for the precise switched overlay timing of a second video signal while insuring an exact integer number of pixels per active horizontal scan line of a composite, interleaved video signal comprised of the first and second video signals.